Method of rolling worm gear and the worm gear

ABSTRACT

A worm is formed by rolling to reduce the number of processing steps while ensuring the worm accuracy satisfactorily. A work material is positioned between a first rolling die ( 100 ) and a second rolling die ( 101 ) by rotation of the rolling dies and a servomotor ( 76 ) for feeding the work material. When the lead angle of a worm is large and there is a large difference between the finished diameter and the blank diameter, the lead angle changes during the progress of rolling process, resulting in occurrence of through-feed. A slide plate ( 55 ) is freely slidable on a slide ( 53 ). Therefore, when through-feed occurs owing to the lead angles of the first and second rolling dies ( 100 ) and ( 101 ) and the workpiece, the slide plate ( 55 ) slides freely, and this is detected with a detecting sensor. The rotation of the first and second rolling dies ( 100 ) and ( 101 ) is stopped. Then, the first and second rolling dies ( 100 ) and ( 101 ) initiate reverse rotation, and at the same time, the slide plate ( 55 ) also initiates movement in the reverse direction. Thus, rolling is started. Thereafter, similar processing is repeated to perform rolling.

TECHNICAL FIELD

[0001] The present invention relates to a method of forming a worm byrolling and also relates to a worm formed by the rolling method. Moreparticularly, the present invention relates to a worm rolling method forforming a worm for driving a steering wheel of an automobile or the likeby rolling. The present invention also relates to a worm formed by therolling method.

BACKGROUND ART

[0002] There is known a type of electric power steering system for avehicle in which the rotational output of an electric motor is reducedin speed through a worm gear mechanism to assist in driving an outputshaft connected to a steering wheel. In a known type of power steeringsystem for light cars to which no heavy load is applied, a combinationof a metallic cylindrical worm and a worm wheel made of a resin is usedas a worm gear mechanism [for example, Japanese Patent ApplicationUnexamined Publication (KOKAI) No. Hei 9-24855].

[0003] Because it requires accuracy, the metallic worm is produced byturning quench-hardened steel in a lathe, followed by heat treatment andfinish grinding. When turned in a lathe, the workpiece is cut with acutting tool. However, when it is desired to increase productivity, aconical milling cutter is used, and the workpiece is cut in a threadcutting manner with the axis of the milling cutter inclined with respectto the worm axis by γ (the lead angle at the pitch line of the worm).

[0004] However, the above-described worm manufacturing process needsroughly at least three steps, i.e. cutting, heat treatment, andgrinding. Regarding equipment therefor, also, at least three apparatusesare required, i.e. a lathe, heat-treatment equipment, and a grindingmachine. For this reason, the processing cost increases, and theadvantage of the resin worm wheel cannot satisfactorily be exhibited.

[0005] Meanwhile, when an external thread, a worm or the like is formedby rolling, a first rolling die and a second rolling die, which aredisposed facing each other, are fed toward each other to penetrate thework material. At this time, when the lead angle of the worm is largeand there is a large difference between the finished diameter and theblank diameter, the workpiece may undesirably travel during the progressof rolling process, causing a change in the lead angle. Such aphenomenon is known as “through-feed”. If through-feed occurs, thecondition of contact of the cylindrical dies becomes different betweenthe flank of thread in the direction of travel of the workpiece due tothe through-feed and the opposite flank, resulting in degradation of thefinished accuracy of the rolled surface. To prevent the occurrence ofthrough-feed, the conventional practice is to make correction bychanging the phase position in the axial direction of the first orsecond rolling die on the basis of visual observation or the like.

[0006] However, this correction method for preventing through-feed isdifficult to carry out for a component part in which there is a shaft oneach side of an external thread or a worm, the shaft having a largerdiameter than that of the external thread or the worm, because therolling die may interfere with the shaft. In such a case, the dies maybe rotated in the forward or reverse direction to perform rolling. Withthis method, however, high product accuracy cannot be obtained owing tothe occurrence of backlash or the like, and productivity is inferior.The present applicant proposed a spindle tilting mechanism that causesthe first and second rolling dies to pivot about an axis perpendicularto the axes of rotation of the dies to prevent the occurrence ofthrough-feed [Japanese Patent Application Unexamined Publication (KOKAI)No. Hei 11-285766].

[0007] However, even if the spindle tilting mechanism is used, theoccurrence of through-feed cannot completely be prevented in the case ofa workpiece for a worm or the like that has a large change in diameterduring the rolling process from the initiation to the termination of theprocess, resulting in occurrence of a working error.

[0008] An object of the present invention is to provide a method offorming a worm by rolling that allows a reduction in the number ofprocessing steps while ensuring the worm accuracy satisfactorily, andalso provide a worm formed by the rolling method.

[0009] Another object of the present invention is to provide a method offorming a worm by rolling that allows a reduction in costs whileensuring the worm accuracy satisfactorily, and also provide a wormformed by the rolling method.

[0010] The advantages of the present invention are as follows. Thepresent invention allows an axially shifted, thin-toothed worm to beformed with high accuracy by rolling. Accordingly, the number ofprocessing steps required can be reduced in comparison to theconventional worms. Moreover, the processing cost can be reduced to aconsiderable extent.

DISCLOSURE OF THE INVENTION

[0011] To attain the above-described objects, the present inventionadopts the following means.

[0012] A worm rolling method according to the present invention iscarried out by using a rolling machine. The rolling machine has aplurality of cylindrical dies for rolling a cylindrical blank placed inthe center between the dies; die rotationally driving means forrotationally driving the dies; blank supporting means for rotatablysupporting the blank; and penetrate feed means for penetrate-feeding thedies toward each other. In the worm rolling method, a worm is formed byrolling by alternately repeating a first step of rolling the blank bypenetrate-feeding the dies toward the blank while synchronously rotatingthe dies in the same direction and a second step of rolling the blankwith the dies by reversing the direction of rotation of the dies aftertermination of the first step.

[0013] In the above-described worm rolling method, if the second step iscarried out after the dies have been withdrawn in the oppositedirections to the penetrate feed directions, rolling of higher accuracycan be performed continuously. To improve the roundness of the worm, theworm should preferably be formed by alternately carrying out the firststep and the second step in a state where the penetrate feed of the diesis suspended.

[0014] If the dies of the rolling machine comprise two dies disposedapproximately in the center between four guides such that the axes ofrotation of the dies are parallel to each other, even more effectiverolling can be performed.

[0015] Preferably, in the worm rolling method, the occurrence ofthrough-feed, which is axial travel of the worm due to a change in thelead angle of the worm caused by a change in the diameter of the blankduring the rolling, is detected from the travel of the blank, and whenthe travel has exceeded a set range, the direction of rotation of thedies is reversed to reverse the direction of the through-feed.

[0016] Preferably, in the worm rolling method, the rolling machine hasspindle tilting means for pivoting the dies about an axis perpendicularto the axes of rotation of the dies. Through-feed, which is axial travelof the blank due to a change in the lead angle caused by a change in thediameter of the blank during the rolling, is detected, and a correctionpivot angle of the spindle tilting means is computed to cancel thethrough-feed. Then, the dies are pivoted by the spindle tilting means byan amount corresponding to the correction pivot angle to cancel thethrough-feed.

[0017] A worm according to the present invention is formed by a wormrolling method using a rolling machine. The rolling machine has aplurality of cylindrical dies for rolling a cylindrical blank placed inthe center between the dies; die rotationally driving means forrotationally driving the dies synchronously with each other; blanksupporting means for rotatably supporting the blank; and penetrate feedmeans for penetrate-feeding the dies toward each other. The bottombetween the teeth of the worm has a vertex as viewed in a sectioncontaining the axis of the worm. Preferably, the vertex has an angle of120 to 150 degrees in the case of carbon steel for mechanical structure,and the tip of the vertex has an arcuate configuration with a radius of1.0 to 1.5 mm as viewed in a section.

[0018] Another worm according to the present invention is formed by aworm rolling method using a rolling machine. The rolling machine has aplurality of cylindrical dies for rolling a cylindrical blank placed inthe center between the dies; die rotationally driving means forrotationally driving the dies synchronously with each other; blanksupporting means for rotatably supporting the blank; and penetrate feedmeans for penetrate-feeding the dies toward each other. The bottombetween the teeth of the worm has an arcuate configuration as viewed ina section containing the axis of the worm. Preferably, the arcuateconfiguration has a radius of 1.0 to 1.5 mm as viewed in a section inthe case of carbon steel for mechanical structure.

[0019] Preferably, the dies of the rolling machine used to roll the wormaccording to the present invention comprise two dies disposedapproximately in the center between four guides such that the axes ofrotation of the dies are parallel to each other.

[0020] Further, the rolling machine used to roll the worm according tothe present invention preferably has spindle tilting means for pivotingthe dies about an axis perpendicular to the axes of rotation of thedies. Preferably, the tooth thickness of the worm is smaller than thetooth space thereof as viewed in a section containing the axis of theworm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a three-dimensional external view showing the whole of arolling machine for rolling a worm according to the present invention.

[0022]FIG. 2 is a partly-sectioned view of the rolling machine as takenalong the line II-II in FIG. 1.

[0023]FIG. 3 is a front view of the rolling machine.

[0024]FIG. 4 is a plan view of the rolling machine.

[0025]FIG. 5 is a left-hand side view of a second die moving plate.

[0026]FIG. 6 is a plan view of the rolling machine, schematicallyshowing the mechanism of a die feed apparatus.

[0027]FIG. 7 is a side view of a work feed apparatus for supporting andfeeding a workpiece, which is an object that is being manufactured.

[0028]FIG. 8(a) is a sectional view of the tooth profile of a worm.

[0029]FIG. 8(b) is a sectional view of the tooth profile of another wormdifferent in the tooth space configuration from that shown in FIG. 8(a).

[0030]FIG. 9 is a block diagram showing the arrangement of a CNC systemand motors for various control operations.

[0031] FIGS. 10(a) and 10(b) are diagrams showing the process sequencefor forming a worm by rolling.

[0032] FIGS. 11(c) and 11(d) are diagrams showing the process sequencefor forming a worm by rolling.

[0033] FIGS. 12(e) and 12(f) are diagrams showing the process sequencefor forming a worm by rolling.

BEST MODE FOR CARRYING OUT THE INVENTION

[0034] Embodiments of the present invention will be described below.FIG. 1 is a three-dimensional external view showing the whole of arolling machine for forming a worm according to the present invention.FIG. 2 is a partly-sectioned view of the rolling machine as taken alongthe line II-II in FIG. 1. FIG. 3 is a front view of the rolling machine.FIG. 4 is a sectional plan view. The rolling machine 1 is acylindrical-die rolling machine for plastically working a blank byplastically deforming it with cylindrical dies disposed facing eachother. In the conventional cylindrical-die rolling machine, a moving diedriven to rotate and a stationary die that is free running with themoving die are disposed facing each other so that the axes of rotationof the dies are parallel to each other. A blank is placed in the middlebetween the dies.

[0035] The rolling machine 1 for forming a worm according to the presentinvention drives two cylindrical dies to rotate in the same directionsimultaneously and synchronously with each other during rolling. Thedetailed structure therefor will be described below. A bed 2 is astructure with a hollow inside that constitutes the body of the rollingmachine 1 (see FIG. 2). The bed 2 is roughly in the shape of a box,which is produced by casting or welding steel plates. Two parallel firstguide rails 3 are secured to the top of the bed 2 with bolts or thelike. A first die moving and supporting plate 4 is movably mountedthrough linear bearing blocks 5 on the two first guide rails 3. A firstdie moving plate 6 is integrally secured to the front of the first diemoving and supporting plate 4. The first die moving plate 6 is alsomovably provided through linear bearing blocks 5 on the two first guiderails 3.

[0036] A first cylindrical die supporting plate 8 is supported on thefront 7 of the first die moving plate 6 in such a manner as to bepivotable for the reason stated later (see FIG. 3). The firstcylindrical die supporting plate 8 has two bearings, i.e. a firstcylindrical die bearing 9 and a second cylindrical die bearing 10,provided thereon at a distance therebetween. A first cylindrical dieshaft 11 is horizontally disposed between the first cylindrical diebearing 9 and the second cylindrical die bearing 10. The first andsecond cylindrical die bearings 9 and 10 support both ends of the firstcylindrical die shaft 11.

[0037] The first cylindrical die supporting plate 8 is pivotable about apivot axis perpendicular to the center axis of the first cylindrical dieshaft 11 through a first spindle tilting mechanism (not shown). Thefirst spindle tilting mechanism comprises a gear disposed on a side ofthe first die moving plate 6 and a servomotor that meshes with the gear.The servomotor 142 for pivotably driving the first cylindrical diesupporting plate 8 controls the pivot angle position of the firstcylindrical die supporting plate 8 under the control of a CNC system 120(see FIG. 9), which will be described later. The first spindle tiltingmechanism prevents the occurrence of a working error due to athrough-feed phenomenon that may occur during working of a componentpart having a helical configuration such as a screw or a worm (describedlater).

[0038] A gear box 12 is disposed at an end of the second cylindrical diebearing 10. The gear box 12 contains a detector 134 (see FIG. 9). A gearmechanism in the gear box 12 transmits rotation from a driving shaft 14to the first cylindrical die shaft 11 through a universal joint 13. Thedriving shaft 14 is further connected to an output shaft (not shown) ofa speed reduction mechanism 16 through a universal joint 15. The speedreduction mechanism 16 is fixedly supported by a bracket 18.

[0039] The bracket 18 is mounted on a driving mechanism supporting plate20. The driving mechanism supporting plate 20 is secured adjacent to thecenter of a side of the bed 2 integrally with the bed 2. The outputshaft of a servomotor 17 is connected to the input shaft (not shown) ofthe speed reduction mechanism 16. Consequently, the rotational output ofthe servomotor 17 is reduced in speed through the speed reductionmechanism 16 and transmitted through the universal joint 15, the drivingshaft 14, the universal joint 13 and the gear mechanism in the gear box12 to rotationally drive the first cylindrical die shaft 11 according toa rotational speed command. The rotational output of the servomotor 17is controlled by the CNC system 120 (described later).

[0040] To transmit the rotational drive of the servomotor 17 to thefirst cylindrical die shaft 11, a rotational drive transmittingmechanism comprising two universal joints 15 and 13 is adopted. Theservomotor 17 is secured to the driving mechanism supporting plate 20,whereas the first cylindrical die shaft 11 is not fixed in positionbecause the first die moving plate 6 is movable along the two firstguide rails 3. Therefore, the rotation cannot smoothly be transmitted tothe first cylindrical die shaft 11 through a conventional jointstructure. The rotational drive transmitting mechanism using theuniversal joints 13 and 15 performs the function of transmitting therotation of the servomotor 17 to the first cylindrical die shaft 11smoothly and at uniform speed.

[0041] On the other hand, a second die moving plate 25 is disposed at aposition on the bed 2 that faces the first die moving plate 6. Twosecond guide rails 26 are secured to the top of the bed 2 with bolts orthe like. The second guide rails 26 are disposed at respective positionson the straight prolongation of the two first guide rails 3 for guidingthe first die moving plate 6 (see FIG. 4). The second die moving plate25 is movably mounted through linear bearing blocks 27 on the two secondguide rails 26 (see FIG. 3).

[0042] A second cylindrical die supporting plate 29 is supported on thefront 28 of the second die moving plate 25 in such a manner as to bepivotable about the center O for the reason stated later (see FIG. 5).The second cylindrical die supporting plate 29 has two bearings, i.e. athird cylindrical die bearing 30 and a fourth cylindrical die bearing31, provided thereon at a distance therebetween (see FIG. 2). A secondcylindrical die shaft 32 is horizontally disposed between the thirdcylindrical die bearing 30 and the fourth cylindrical die bearing 31.The third and fourth cylindrical die bearings 30 and 31 rotatablysupport both ends of the second cylindrical die shaft 32.

[0043] The second cylindrical die supporting plate 29 is pivotable aboutthe pivot axis O perpendicular to the center axis of the secondcylindrical die shaft 32 by an angle +α or −α (see FIG. 5) through asecond spindle tilting mechanism (not shown). The second spindle tiltingmechanism comprises a gear disposed on a side of the second die movingplate 25 and a servomotor that meshes with the gear. The servomotor 147for driving the second cylindrical die supporting plate 29 controls theposition of the second cylindrical die supporting plate 29 under thecontrol of the CNC system 120 (see FIG. 9), which will be describedlater. The second spindle tilting mechanism prevents the occurrence ofan error due to a through-feed phenomenon that may exert an influenceupon the configuration accuracy of a component part having a helicalstructure such as a screw or a worm (described later).

[0044] A gear box 33 is disposed at an end of the fourth cylindrical diebearing 31. The gear box 33 contains a rotation detecting mechanism (notshown). A gear mechanism in the gear box 33 transmits rotation from adriving shaft 35 to the second cylindrical die shaft 32 through auniversal joint 34. The driving shaft 35 is further connected to anoutput shaft (not shown) of a speed reduction mechanism 37 through auniversal joint 36. The speed reduction mechanism 37 is fixedlysupported by the above-described bracket 18.

[0045] The output shaft of a servomotor 38 is connected to the inputshaft of the speed reduction mechanism 37. Consequently, the rotationaloutput of the servomotor 38 is reduced in speed through the speedreduction mechanism 37 and transmitted through the universal joint 36,the driving shaft 35, the universal joint 34 and the gear mechanism inthe gear box 33 to control the second cylindrical die shaft 32 accordingto a command from the CNC system 120 (see FIG. 9).

[0046] The first die moving and supporting plate 4 and the first diemoving plate 6 have shaft securing portions 41 provided in the fourcorners of the outer periphery thereof. One end of each of fourconnecting shafts 40 is secured to one of the shaft securing portions41. The four connecting shafts 40 are disposed parallel to each otherand also parallel to the first guide rails 3 and the second guide rails26. The second die moving plate 25 has guide portions 42 provided in thefour corners of the outer periphery thereof. The second die moving plate25 is movably supported by the four connecting shafts 40 throughbearings incorporated in the guide portions 42.

[0047] [Die Feed Apparatus 49]

[0048]FIG. 6 is a schematic plan view of the rolling machine,schematically showing the mechanism of the die feed apparatus. As willbe understood from the foregoing description, the second die movingplate 25 is movable relative to the first die moving plate 6 toward oraway from it by being guided by the second guide rails 26 and the fourconnecting shafts 40. The other ends of the connecting shafts 40 areconnected and secured to a pressure plate 45. A hydraulic cylinder 50comprising a hydraulic cylinder device is secured to the pressure plate45. The hydraulic cylinder 50 has a servovalve capable of controllingthe extension position of the piston with high accuracy. The hydrauliccylinder 50 has a piston rod 51 as an output shaft. The distal end ofthe piston rod 51 is secured to the back 52 of the second die movingplate 25.

[0049] When the hydraulic cylinder 50 is driven by introducing ahydraulic pressure thereinto, the piston rod 51 extends. Because thehydraulic cylinder 50 is secured to the pressure plate 45 and thepressure plate 45 is connected to the first die moving plate 6 throughthe connecting shafts 40, the first die moving plate 6 and the seconddie moving plate 25 approach each other as the piston rod 51 extends.

[0050] As shown in FIG. 6, a rack 91 is disposed to face in the samedirection as the direction of movement of the second die moving plate25. One end of the rack 91 is secured to the second die moving plate 25.One end of a rack 90 is secured to the pressure plate 45. The racks 90and 91 are disposed parallel to each other.

[0051] The racks 90 and 91 are in mesh with a pinion 93. A pinion shaft94 of the pinion 93 is rotatably provided on the bed 2. Consequently,when the hydraulic cylinder 50 is driven, the piston rod 51 extends.Because the hydraulic cylinder 50 is secured to the pressure plate 45and the pressure plate 45 is connected to the first die moving plate 6through the connecting shafts 40, the first die moving plate 6 and thesecond die moving plate 25 come toward or away from each other accordingto the extension of the piston rod 51.

[0052] At this time, the pinion shaft 94 does not move although itrotates because the pinion shaft 94 is rotatably supported on the bed 2.Consequently, the center position of the space between the first diemoving plate 6 and the second die moving plate 25 is always located at afixed position on the bed 2. If the center axis of the workpiece (objectthat is being manufactured) is set coincident with the fixed position,the working accuracy of the workpiece increases, and it also becomeseasy to feed the workpiece to the rolling machine 1 and to remove ittherefrom.

[0053] The space between the first die moving plate 6 and the second diemoving plate 25 is measured by a moving plate space measuring means(described later) disposed on the second die moving plate 25. The movingplate space measuring means comprises a linear scale 111 secured to thetop of the second die moving plate 25, a sensor (not shown) for readingmagnetic graduations on the linear scale 111, a bar-shaped sensor plate110 having the sensor secured thereto, and so forth (see FIG. 4).

[0054] One end of the sensor plate 110 is secured to the first diemoving plate 6. Accordingly, when the first die moving plate 6 and thesecond die moving plate 25 move relative to each other, the sensor readsthe magnetic graduations on the linear scale 111. Thus, it is possibleto read the distance between the first die moving plate 6 and the seconddie moving plate 25. The rotational drive of the first and secondcylindrical die shafts 11 and 32 in the rolling machine 1 and themovement of each rolling die are controlled synchronously orasynchronouslv by the CNC system 120 (described later).

[0055] [Work Feed Apparatus 59]

[0056]FIG. 7 is a side view of a work feed apparatus 59 for supportingand feeding a workpiece W, which is an object that is beingmanufactured. A first rolling die 100 and a second rolling die 101 arekeyed on the above-described first and second cylindrical die shafts 11and 32, respectively. A single linear rail 58 is secured to the top of abase plate 61 with bolts. Two linear blocks 57 are movably mounted onthe linear rail 58.

[0057] Work length adjusting plates 89 are stretched between both sidesof the two linear blocks 57 and fixedly connected to each other withbolts. Further, a slide retainer 54 is secured to the top of each worklength adjusting plate 89 with bolts. A slide plate 55 is slidablyprovided between the two slide retainers 54. The slide plate 55, whichis guided by the slide retainers 54, moves during the rolling process.The slide plate 55 is movable only within a set range.

[0058] Thus, a slide 53 is formed from the linear blocks 57, the worklength adjusting plates 89, the slide retainers 54 and the slide plate55. A tailstock 60 is secured to the top of the slide plate 55. A center62 for rotatably supporting a worm blank or a workpiece W is insertedinto and secured to the tailstock 60. A sensor dog 63 for determiningthe movable range of the slide plate 55 is secured to the tailstock 60with bolts 64. An advanced position detecting sensor 79 for sensing themovement of the sensor dog 63 is secured to the base plate 61.

[0059] A chuck plate 65 is secured to the slide plate 55 so as to facethe tailstock 60. The chuck plate 65 is provided with a centersupporting shaft 67. A live center 66 is rotatably supported by thecenter supporting shaft 67 through a bearing 68. On the other hand, apneumatic chucking cylinder 69 for axially movably driving the centersupporting shaft 67 is secured to the back of the chuck plate 65 withbolts 70. The pneumatic chucking cylinder 69 has a piston rod that isactuated by pneumatic pressure. The piston rod is connected to thecenter supporting shaft 67.

[0060] Accordingly, when the pneumatic chucking cylinder 69 isactivated, the center supporting shaft 67 and the live center 66 aredriven to extend or contract in the axial direction, thereby allowingthe blank or the workpiece W to be loaded between the live center 66 andthe center 62 or unloaded therefrom. A nut 72 is secured to the linearblock 57. The nut 72 has a feed screw 73 screwed therein. One end of thefeed screw 73 is connected to an output shaft 77 of a servomotor 76through a coupling 74, which is a rotary joint.

[0061] Accordingly, when the servomotor 76 is driven, the feed screw 73is driven to rotate, causing the slide 53 to move along the linear rail58. Thus, the position of the slide 53 is controlled. A sensor dog 71 issecured to the top of the chuck plate 65 with bolts 78. A retractedposition detecting sensor 75 for sensing the movement of the sensor dog71 is secured to the base plate 61. Accordingly, the advanced positiondetecting sensor 79 detects the advanced position of the workpiece W,whereas the retracted position detecting sensor 75 detects the retractedposition of the workpiece W.

[0062] A linear scale 99 is secured to a side of the chuck plate 65. Thelinear scale 99 is a magnetic scale. The movement of the linear scale 99is read and sensed by a fixed reader (not shown). Accordingly, themovement of the workpiece W is detected by sensing the movement of thelinear scale 99.

[0063] As the rolling process progresses, the lead angle of theworkpiece W, which is a worm, changes, resulting in an error, as hasbeen stated above. If this error does not occur, the workpiece W willnot move in the axial direction in theory, as will be described later.The error appears as an amount of through-feed by which the workpiecetravels in the axial direction. The movement of linear scale 99 is readby the sensor, and the read data is computed as an amount ofthrough-feed by a through-feed detecting program 160 (see FIG. 9).

[0064] That is, when a helical groove is to be formed on the workpiece Wby rolling, the first and second rolling dies 100 and 101 are fed towardeach other little by little so as to penetrate the workpiece W. As thefirst and second rolling dies 100 and 101 are fed to penetrate theworkpiece W in this way, the minor diameter of the workpiece Wdecreases. Therefore, the root circumferential length of the workpiece Wbecomes shorter at the time of completion of penetrating the first andsecond rolling dies 100 and 101 into the workpiece W than the rootcircumferential length at the time of initiating the penetrateoperation. In other words, the relationship between the circumferentiallength of the workpiece W and the pitch differs between the time ofinitiation of the rolling process and the time of termination of therolling process.

[0065] The circumferential length of the workpiece W shortens during thepenetrate operation as follows. Assuming that the circumferential lengthat the time of initiating the penetrate feed operation is L and the leadangle (helix angle) of the worm is β and constant, the circumferentiallength L has shortened geometrically by an amount corresponding to areduction in the diameter upon completion of the penetrate feedoperation. In the conventional rolling machine, however, the lead angle(helix angle) β of the cylindrical dies is invariable during thepenetrate feed operation. Therefore, there is a pitch deviation δPbetween the pitch P of the workpiece W at the time of initiating thepenetrate feed operation and the pitch P1 of the workpiece W at the timeof termination of the penetrate feed operation.

[0066] During the rolling process, the workpiece W travels in thedirection of its center axis by a distance corresponding to the pitchdeviation δP. The phenomenon in which the workpiece W travels during therolling process is known as “through-feed” of the workpiece W. Thethrough-feed phenomenon occurs particularly remarkably in the case of aworkpiece W such as a worm or a screw having large major and minordiameters. If through-feed occurs, the flank of the worm or the threadin the direction of movement of the workpiece W due to the through-feedstrongly contacts the first and second rolling dies 100 and 101, whichare cylindrical dies. As a result, the configuration accuracy, i.e.working accuracy, of the rolled component part is degraded.

[0067] The above-described first and second spindle tilting mechanismsare provided to prevent the occurrence of through-feed that exerts aninfluence upon the configuration accuracy of products such as a worm anda screw. That is, the first and second spindle tilting mechanismscorrect the lead angle on the basis of the circumferential length of theworkpiece during the penetrate feed operation. It should be noted thatthe lead angle of the workpiece being rolled is changed by thecorrection, but the change of the lead angle is only slight and hencehas no significant effect on the configuration accuracy. The workingerror is less than in the case where the lead angle is not corrected,and falls within the tolerances satisfactorily.

[0068] [CNC System 120]

[0069]FIG. 9 is a block diagram showing the arrangement of the CNCsystem 120 and motors for various control operations. The CNC system 120can use an NC special-purpose machine or a so-called personal computerNC system having a numerical control function and a personal computerfunction, which comprises a personal computer and an NC board or thelike for performing servomotor control, sequence control, etc. that isinstalled in an expansion slot of the personal computer. The CNC system120 is provided with a CPU 121 as an information processing means forperforming various data processing. The CPU 121 is connected with aflash memory 123 as a main memory unit and a RAM 124 through a bus 122.

[0070] The CPU 121 operates according to a system program and datastored in the flash memory 123 and a program loaded (read) into the RAM124, together with data read thereinto. Programs that may be loaded intothe RAM 124 include an OS (Operating System) as a basic program, an NCcommand processing program for performing processing according to eachof many NC commands, a tool-work data setting program, a measureddimension computing program 125, a tilt angle computing program 126, ameasurement data setting program 127, a through-feed detecting program160, and a display control program for displaying characters and figureson a display unit 130.

[0071] The measured dimension computing program 125 is for executingcomputing at every moment to determine the penetrate feed rate forrolling from the space between the first and second rolling dies 100 and101, which is read from the linear scale 111 with the sensor, and datadetected with the detectors 134, 137, etc. The result of the computationis given as a command to a die space control unit 150 by an NC workingprogram in an NC working program memory 129 and thus executed.

[0072] The tilt angle computing program 126 is for computing the tiltangles of the first and second spindle tilting mechanisms at everymoment from the amount of through-feed measured by the through-feeddetecting program 160 and the space between the first and second rollingdies 100 and 101 measured by the measured dimension computing program125. The result of the computation is given as a command to a tiltcontrol unit 140 for the first spindle tilting mechanism and to a tiltcontrol unit 145 for the second spindle tilting mechanism by the NCworking program in the NC working program memory 129. The measurementdata setting program 127 is for storing data for setting the dimensionsof the first and second rolling dies 100 and 101, the dimension of theworkpiece W, etc. into the memory.

[0073] The through-feed detecting program 160 is for measuring theamount of through-feed from data obtained by reading the linear scale99. The CPU 121 is connected with a parameter memory 128 through the bus122. The parameter memory 128 has previously been stored with variousparameters necessary for the rolling process. If a nonvolatile memory isused therefor, the parameter memory 128 can retain the memory contentseven if the power supply of the CNC system 120 is turned off.

[0074] Further, the CPU 121 is connected with an NC working programmemory 129 and so forth through the bus 122. The NC working programmemory 129 has been stored with an NC working program for performingrolling by sequentially controlling the movement of the workpiece W tothe working position or to the withdrawn position, the speed and amountof rotation of the first and second rolling dies 100 and 101 to roll theworkpiece W, the tilt angle of the first and second spindle tiltingmechanism, the space between the two dies, and so forth. The NC workingprogram is prepared by the operator by programming the workingconditions, the rotational speed, travel speed and feed rate of eachdie, etc. according to the kind, material and configuration of theworkpiece W.

[0075] In addition, the CPU 121 is connected with input/output devicesthrough the bus 122. As the input/output devices, a display unit 130 fordisplaying characters and figures and an input unit 131 for the operatorto enter data are connected to the bus 122 through an interface circuit.As the display unit 130, a CRT, an EL display panel, a liquid crystaldisplay, etc. can be used. As the input unit 131, a keyboard or a touchpanel combined with the display unit 130 as one unit can be used.

[0076] Further, the CPU 121 may be connected with a fixed disk unit asan auxiliary storage unit through the bus 122. In such a case, the fixeddisk unit has previously been stored with various programs or the liketo be executed by the CPU 121, and as occasion demands, these programsor the like are appropriately loaded into the RAM 124 or the NC workingprogram memory 129 from the fixed disk unit.

[0077] The CNC system 120 is connected to the servomotor 17 forrotationally driving the first rolling die 100 through a spindlerotation control unit 132 for the first rolling die 100 and furtherthrough an amplifier 133. The rotational speed of the servomotor 17 isfed back to the amplifier 133 through the detector 134 to maintain apredetermined rotational speed. Accordingly, the angle position of thefirst rolling die 100 about the axis thereof is fed back to the spindlerotation control unit 132 from the detector 134, thereby allowing thefirst rolling die 100 to be controlled at a desired rotation speed and adesired angle position.

[0078] Similarly, the CNC system 120 is connected to the servomotor 38for rotationally driving the first rolling die 100 through a rotationcontrol unit 135 for the second rolling die 101 and further through anamplifier 136. The rotational speed of the servomotor 38 is fed back tothe amplifier 136 through the detector 137 to control the servomotor 38at a predetermined rotational speed. Accordingly, the angle position ofthe second rolling die 101 about the axis thereof is fed back to thespindle rotation control unit 132 from the detector 137, therebyallowing the second rolling die 101 to be controlled at a desiredrotation speed and a desired angle position.

[0079] Further, the CNC system 120 is connected to a servomotor 142 ofthe first spindle tilting mechanism for tilting the first rolling die100 and also connected to a servomotor 147 of the second spindle tiltingmechanism for tilting the second rolling die 101 to control theservomotors 142 and 147. More specifically, the CNC system 120 isconnected to the servomotor 142 for controlling the pivoting of thefirst rolling die 100 through a tilt control unit 140 for the firstrolling die 100 and further through an amplifier 141. The rotation ofthe servomotor 142 is fed back to the amplifier 141 through a detector143 to maintain a predetermined tilt angle. Accordingly, the pivot angleposition for tilting the first rolling die 100 is fed back to the tiltcontrol unit 140 from the detector 143. Thus, the first rolling die 100can be positioned at a desired pivot angle position for tilting it.

[0080] Similarly, the CNC system 120 is connected to the servomotor 147for controlling the pivot angle position for tilting the second rollingdie 101 through a tilt control unit 145 for the second rolling die 101and further through an amplifier 146. The rotation of the servomotor 147is fed back to the amplifier 146 through a detector 148 to control thesecond rolling die 101 at a predetermined tilt angle. Accordingly, thepivot angle position of the second rolling die 101 is fed back to thetilt control unit 145 from the detector 148. Thus, the second rollingdie 101 can be positioned at a desired pivot angle position for tiltingit.

[0081] Further, the CNC system 120 controls the space between the firstand second rolling dies 100 and 101 by on-off controlling the valveopening and closing operation of a servovalve 152 of the hydrauliccylinder 50. For this purpose, the CNC system 120 is connected to theservovalve 152 for controlling the hydraulic cylinder 50 through a diespace control unit 150 and an amplifier 151. In this example, thepositioning accuracy of the system is of the order of 4 to 5 μm under aload of 100 KN.

[0082] The CNC system 120 is connected to the servomotor 76 forcontrolling the position of the workpiece W through a positioningcontrol unit 153 of the work feed apparatus 59 and further through anamplifier 154. The rotation of the servomotor 76 is fed back to theamplifier 154 through a detector 155, and thus the workpiece W is fed toa predetermined rolling initiation position. The retracted positiondetecting sensor 75 is for sensing the most retracted position of theworkpiece W during the rolling process. The advanced position detectingsensor 79 is for sensing the most advanced position of the workpiece Wduring the rolling process. The above-described driving shafts arecontrolled independently of each other except the synchronous rotationof the first and second rolling dies 100 and 101. It should be noted,however, that the driving shafts may be controlled synchronously whenthe system has control capability to spare for the synchronous control.

[0083] The tailstock 60 has the linear scale 99 disposed on a sidethereof, as has been stated above. The through-feed detecting program160 is for computing the amount of through-feed after the initiation ofrolling process by reading the linear scale 99. The through-feeddetecting program 160 computes the amount of through-feed of theworkpiece W after the initiation of rolling process in a state where theworkpiece W is clamped between the tailstock 60 and the chuck plate 65.

[0084] [Worm Rolling Method 1]

[0085] A method of forming a worm by rolling in the above-describedrolling machine 1 and with the work feed apparatus will be describedwith regard to a worm used in a worm gear mechanism for assisting anelectric power steering system for a vehicle, by way of example. FIG.8(a) is a sectional view of the tooth profile of a worm as taken along aplane containing the center line of the worm. The worm 80 is formed sothat the tooth thickness 82 is smaller than the tooth space 83 incomparison to ordinary worms, as viewed at a circumferential position ina section 81 containing the pitch line.

[0086] The reason for this is to increase the tooth thickness of a wormwheel (not shown) that meshes with the worm 80 because the worm wheel ismade of a synthetic resin and hence weak in mechanical strength. Thetooth bottom is formed with an angle α so that two helical slantsurfaces 84 intersect each other as viewed in a section. In thisexample, the angle α is 150 degrees. In the worm rolling methodaccording to the present invention, the angle α is preferably in therange of from 120 to 150 degrees from the viewpoint of smoothing theplastic flow of material, provided that carbon steel for mechanicalstructure (e.g. S45C) is used in the manufacture of a worm having apitch circle of 18 to 20 mm and a module of 1.5 to 2.0 and in which thenumber of threads is 2 or 3. It is also preferable that the tooth depthshould be not more than 6 mm and the minor diameter should be not lessthan 10 mm. If the dimensions are not within these numerical valueranges, such a phenomenon that the material separates from the surfacemay occur.

[0087] The two slant surfaces 84 intersect each other with the angle αin the middle of the bottom. Thus, the middle of the bottom has thesmallest diameter. The tooth surface 88 and each slant surface 84 areconnected to each other through a curved surface 85. The two slantsurfaces 84, i.e. conical surfaces, are also connected through a curvedsurface 86. The curved surface 86 of the worm of the above-describedspecifications should preferably have a radius of 1.0 to 1.5 mm asviewed in a section from the viewpoint of smoothing the plastic flow ofmaterial. The bottom has a tapered convex configuration as viewed fromthe tool side, which allows the material to plastically flow easily anduniformly. Accordingly, rolling can be effected smoothly. Morespecifically, the bottom has a configuration in which two slant surfaces84, which are conical surfaces, intersect each other. In other words,the bottom has an angular tip. Therefore, the metal is distributed alongboth sides of the angular tip, thus allowing the material to plasticallyflow easily and uniformly.

[0088]FIG. 8(b) is a sectional view of the tooth profile of another wormdifferent in the tooth space configuration from that shown in FIG. 8(a). The bottom 87 of the tooth space 83 shown in FIG. 8(b) has aconfiguration defined by a circular arc with a radius R1 as viewed in asection. The bottom 87 shown in FIG. 8(b) has an arcuate configuration,i.e. a convexly arcuate configuration as viewed from the rolling dieside. Therefore, the worm shown in FIG. 8(b) has the advantage that thematerial can plastically flow easily and naturally in comparison to thebottom configuration shown in FIG. 8(a). In the case of carbon steel formechanical structure, the radius R1 of the bottom 87 should preferablybe in the range of from 1.0 to 1.5 mm as viewed in a section from theviewpoint of smoothing the plastic flow of material.

[0089] FIGS. 10(a) to 12(f) show the process sequence for forming a wormby rolling. When a solid cylindrical blank material M as a workpiece Wis placed in a chucking position, the pneumatic chucking cylinder 69 isactivated to hold the material M between the center 62 and the livecenter 66. During the suspension of rotation of the first and secondrolling dies 100 and 101, the servomotor 76 is started to rotate thefeed screw 73, thereby driving the slide plate 55 to feed the material Mtoward the first and second rolling dies 100 and 101 [see FIG. 10(a)]and further to the rolling initiation position [see FIG. 10(b)].

[0090] The first and second rolling dies 100 and 101 have been set inpredetermined positions, respectively. The first and second rolling dies100 and 101 are started to rotate in the same direction synchronouslywith each other. While the first and second rolling dies 100 and 101 arerotating synchronously, the hydraulic cylinder 50 is driven topenetrate-feed the dies 100 and 101 toward each other. With thispenetrate feed operation, rolling is initiated.

[0091] In the rolling process, if the lead angle (usually the lead angleat the pitch point) of the worm to be manufactured is large and thedifference between the finished diameter and the blank diameter islarge, the lead angle changes during the progress of rolling between therolling initiation position and the rolling termination position,resulting in an error. This error is the above-described through-feed.When the rolling process is initiated, the through-feed occurs, causingthe slide plate 55 to move. The slide plate 55 is freely slidable on theslide 53. Therefore, even if the rotation of the first rolling die 100and that of the second rolling die 101 are controlled strictly so as tobe synchronous with each other, because the lead angle of each of thefirst and second rolling dies 100 and 101 is fixed, the workpiece W thatis being rolled travels in the axial direction by a distancecorresponding to the lead angle of the worm and the amount of movementcorresponding to the number of revolutions of the first and secondrolling dies 100 and 101, i.e. by a distance corresponding to the amountof through-feed.

[0092] When the slide plate 55 has been fed by a predetermined amountowing to the through-feed as the rolling process progresses, the sensordog 63 is detected by the advanced position detecting sensor 79.Alternatively, the detection of the advanced position of the workpiece Wdue to the through-feed may be effected by using the linear scale 99.When this is detected, the first and second rolling dies 100 and 101stop rotating, and the penetrate feed operation by the hydrauliccylinder 50 is stopped. Further, the first and second rolling dies 100and 101 retract away from each other in the opposite directions to thepenetrate feed directions. In this example, the first and second rollingdies 100 and 101 are retracted through a distance of the order of about0.05 to 0.2 mm, i.e. retracted to such an extent that the plungingpressure for rolling is removed, thereby releasing the material M.

[0093] The retracting operation is carried out to release the elasticdeformation of the material M and the elastic deformation of themechanical system of the rolling machine, thereby preventing the firstand second rolling dies 100 and 101 from contacting the material M (theretracting operation will hereinafter occasionally be referred to as“springback”). Thereafter, the first and second rolling dies 100 and 101are fed again to the rolling position (penetrate operation), and reverserotation is started [see FIG. 11(d)]. The penetrate feed operation(penetrate operation) employs a method wherein penetrate feed iseffected while being varied stepwisely in 5 to 30 steps in the case ofthe worm of the above-described specifications. If necessary, a dowelprocess (described later) is performed intermittently at any step of therolling process, as will be described later.

[0094] The rolling process effected by the reverse rotation also allowscorrection of a working error due to through-feed. The detailedmechanism of the error correcting principle is unclear. It is presumed,however, that the error is corrected because the contact between theworkpiece W and the dies is made uniform. This rolling process causesthe workpiece W to travel in an axial direction opposite to the axialdirection in which the workpiece W travels during the first-describedrolling process, in accordance with the lead of the first and secondrolling dies 100 and 101. Then, the sensor dog 71 is sensed by theretracted position detecting sensor 75. Alternatively, the detection ofthe retracted position of the workpiece W due to the through-feed may beeffected by using the linear scale 99.

[0095] Next, the rotation of the first and second rolling dies 100 and101 is stopped, and the hydraulic cylinder 50 is driven to separate thefirst and second rolling dies 100 and 101 from the workpiece W and toretract them to the respective withdrawn positions [see FIG. 12(e)]. Theservomotor 76 is started to rotate the feed screw 73 reversely, therebydriving the slide 53 to feed the workpiece W away from the first andsecond rolling dies 100 and 101 to the previous working initiationposition [see FIG. 10(f)]. Thereafter, the same process is repeated toperform rolling. In the case of a worm of the above-describedspecifications, the rolling process is repeated 15 to 50 times in termsof the number of times of forward/reverse rotation with the first andsecond rolling dies 100 and 101 rotated at a speed of 10 to 40 rev/min.

[0096] When the first and second rolling dies 100 and 101 initiaterolling of the material M, the contact of the dies 100 and 101 with thematerial M is not uniform at all the outer peripheral positions of thematerial M. That is, a worm is usually formed with two or three threads,and the number of combinations of teeth of the first and second rollingdies 100 and 101 that mesh simultaneously with the teeth of the wormdiffers according to the angle position of the outer periphery. In otherwords, because the first and second rolling dies 100 and 101 pressagainst the outer periphery of the material M with a fixed force, theplastic flow differs according to the angle position of the outerperiphery. Consequently, in the case of a worm with two threads, thesectional configuration is likely to become elliptical. In the case of aworm with three threads, the sectional configuration is likely to becomesubstantially triangular.

[0097] To correct the undesired configuration, rolling is performed 2 to5 times in terms of the number of times of forward/reverse rotation in astate where the penetrate feed operation (feed operation) effected bythe hydraulic cylinder 50 is stopped to suspend the penetrate operationby the first and second rolling dies 100 and 101 approaching each other(i.e. dowel step). By doing so, the pitch cylinder and the cylinderdefining the major diameter, which would otherwise be likely to becomeelliptical or substantially triangular in sectional configuration asstated above, is rolled to a round configuration. If the dowel step isnot carried out, the worm of the above-described specifications isformed into an irregular shape with a working error of 0.2 to 0.3 mm interms of the pitch cylinder diameter or the major diameter. With theabove-described rolling method, the worm of the above-describedspecifications was capable of being rolled with a working error within0.02 mm. The dowel operation, which is carried out 2 to 3 times, isperformed intermittently at any step of the penetrate feed operation(penetrate operation) during the rolling process.

[0098] [Worm Rolling Method 2]

[0099] In the above-described worm rolling method 1, the correction ofan error due to the through-feed of the workpiece W is made by reverselyrotating the first and second rolling dies 100 and 101. According to theworm rolling method 2, however, the tilt of the first and second rollingdies 100 and 101 is controlled by controlling the servomotors 142 and147 according to the result of the computation executed by the tiltangle computing program 126, thereby correcting the through-feed. Thus,it is possible to adopt a method whereby rolling is progressed whileeffecting control so that through-feed will not occur, or the amount ofthrough-feed will be less than a predetermined value.

Field of Industrial Application

[0100] The present invention is not limited to the above-described wormfor use in an electric power steering system for an automobile and tothe method of forming the worm by rolling, but may be applied to ametallic worm for power transmission that may be used for otherindustrial machines and consumer machines and also applied to a methodof forming the worm by rolling.

What is claimed is:
 1. A worm rolling method carried out by using arolling machine, said rolling machine having: a plurality of cylindricaldies for rolling a cylindrical blank placed in a center between saiddies; die rotationally driving means for rotationally driving said dies;blank supporting means for rotatably supporting said blank; andpenetrate feed means for penetrate-feeding said dies toward each other;wherein a worm is formed by rolling by alternately repeating: a firststep of rolling said blank by penetrate-feeding said dies toward saidblank while synchronously rotating said dies in a same direction; and asecond step of rolling said blank with said dies by reversing thedirection of rotation of said dies after termination of said first step.2. A worm rolling method according to claim 1, wherein said second stepis carried out after said dies have been withdrawn in oppositedirections to directions of penetrate feed by said penetrate feed means.3. A worm rolling method according to claim 1 or 2, wherein said worm isformed by alternately carrying out said first step and second step in astate where said penetrate feed of said dies is suspended.
 4. A wormrolling method according to claim 1 or 2, wherein said dies of saidrolling machine comprise two dies disposed approximately in a centerbetween four guides such that axes of rotation of said dies are parallelto each other.
 5. A worm rolling method according to claim 1 or 2,wherein through-feed, which is axial travel of said worm due to a changein a lead angle of said worm caused by a change in a diameter of saidblank during said rolling, is detected from travel of said blank, andwhen said travel has exceeded a set range, the direction of rotation ofsaid dies is reversed to reverse a direction of said through-feed.
 6. Aworm rolling method according to claim 4, wherein said rolling machinehas spindle tilting means for pivoting said dies about an axisperpendicular to the axes of rotation of said dies, and whereinthrough-feed, which is axial travel of said blank due to a change in alead angle caused by a change in a diameter of said blank during saidrolling, is detected, and a correction pivot angle of said spindletilting means is computed to cancel the through-feed, and then, saiddies are pivoted by said spindle tilting means by an amountcorresponding to said correction pivot angle to cancel saidthrough-feed.
 7. A worm formed by a worm rolling method using a rollingmachine, said rolling machine having: a plurality of cylindrical diesfor rolling a cylindrical blank placed in a center between said dies;die rotationally driving means for rotationally driving said diessynchronously with each other; blank supporting means for rotatablysupporting said blank; and penetrate feed means for penetrate-feedingsaid dies toward each other; wherein a bottom between teeth of said wormhas an arcuate vertex as viewed in a section containing an axis of saidworm.
 8. A worm formed by a worm rolling method using a rolling machine,said rolling machine having: a plurality of cylindrical dies for rollinga cylindrical blank placed in a center between said dies; dierotationally driving means for rotationally driving said diessynchronously with each other; blank supporting means for rotatablysupporting said blank; and penetrate feed means for penetrate-feedingsaid dies toward each other; wherein a bottom between teeth of said wormhas an arcuate configuration as viewed in a section containing an axisof said worm.
 9. A worm according to claim 7 or 8, wherein said dies ofsaid rolling machine comprise two dies disposed approximately in acenter between four guides such that axes of rotation of said dies areparallel to each other.
 10. A worm according to claim 7 or 8, whereinsaid rolling machine has spindle tilting means for pivoting said diesabout an axis perpendicular to axes of rotation of said dies.
 11. A wormaccording to claim 7 or 8, wherein a tooth thickness of said worm issmaller than a tooth space thereof as viewed in a section containing theaxis of said worm.